As attenuator circuits that include transistor elements are called upon to operate into the microwave and millimeter wave frequency ranges over broader bandwidths, the lumped capacitance of the transistors becomes increasingly difficult to tolerate. At frequencies below a few gigahertz the capacitance can be neglected by selecting a process and transistor design that produces a sufficiently small capacitance. Alternatively, when only a narrow bandwidth is required, then the capacitance can be absorbed into a reactive matching network. However, in transistors operating across multi-octave bandwidths above a few gigahertz, then neither of the preceding solutions is very effective.
To address this problem, the distributed amplifier was developed. A distributed transistor structure is realized by dividing the transistor periphery into an array of smaller devices separated by inductors. These inductors are often realized by narrow width (high impedance) transmission lines. The transmission lines and transistors are arranged in a ladder configuration that forms a synthetic transmission line. The result is a system that advantageously absorbs the transistor capacitance into a broadband transmission line-like structure that can efficiently handle the necessary frequency range. Since a synthetic transmission line can operate from frequencies of 0 Hz up to some very high cutoff frequency, systems designed around the distributed amplifier approach can achieve virtually an infinite amount of octave bandwidth.
In passive applications such as switches and attenuators, the distributed approach shows up again as a preferred way to achieve broad bandwidths at high frequencies in the presence of significant transistor capacitance. The distributed topologies appear in such circuits where shunt transistors are needed, and they take the form of series high impedance line segments separated by shunt transistors.
FIG. 1 shows an exemplary prior art variable attenuator 100 incorporating a distributed transistor structure. Attenuator 100 includes a first series transistor 110, a distributed shunt transistor structure 120, a second series transistor 130, a first gate resistor 115, a second gate resistor 135, and shunt gate resistors 142, 144, 146 and 148. Distributed shunt transistor structure 120 includes a plurality of shunt transistors 122, 124, 126 and 128 separated by series inductors 121, 123, 125, 127 and 129. As explained above, shunt transistors 122, 124, 126 and 128 and series inductors 121, 123, 125, 127 and 129 form a synthetic transmission line, with the transistor capacitances being absorbed therein.
Attenuator 100 is a “T-type attenuator” structure. Series transistor 110, distributed shunt transistor structure 120, and second series transistor 130 each acts as a variable impedance according to the drive voltages supplied to the gates of the respective transistors. An RF, microwave, or millimeter wave signal to be attenuated is input to a first terminal (e.g., a drain) of first series transistor 110 and an attenuated signal is output from a second terminal (e.g., a source) of second series transistor 130. The operation of attenuator 100 is well-understood by those of skill in the art.
However, a principle weakness of the distributed approach relates to the synthetic transmission line itself. There is always a residual passband ripple, the amplitude of which is determined by the upper cutoff frequency and the number of sections in the synthetic transmission line. That is, the passband ripple can be improved, but doing so requires the addition of more sections to the synthetic transmission line. However, the number of sections is limited by the space available for laying out the circuit. Accordingly, a compromise is forced between bandwidth, ripple, and layout size, and the results are not always satisfactory.
What is needed, therefore, is an attenuator that can provide wideband, high attenuation without significant passband ripple. What is also needed is an attenuator with wideband, high frequency amplification capability that can be fabricated with a smaller size.